Correct prop for my Yam. 50 on a K.W.152CC

[bklong]

I have a factory rigged 2007, 50hp 2 stroke Yamaha on my Key West 152CC. This boat weighs about 1100# empty. It takes almost 15 seconds to get up onto a plane with 2 of us onboard, then only tachs up to around 4950RPM at full throttle. Current prop is a Yamaha Stainless 10.5" X13" pitch. Should I try to go down 1" or 2" in pitch, and should I change the diameter? Top operating RPM for this motor is 4500-5500, according to Yamaha specs.

bklong

 

[steelespike]

Re: Correct prop for my Yam. 50 on a K.W.152CC

What speed at 4950?Generally you don't mess with diameter except for very specific problems.You need to be close to the max rated rpm at wot(wide open throttle)
I think your allready with a pretty small pitch.Approaching that for a pontoon.
I think you need to check your setup.Do you trim all the way in for hole shot?Motor in excellent tune,Prop excellent condition,Throttle plates in carb opening all the way.Bottom clean,Motor at max height. Usually the antiventplate(just above the prop) a little higher than the bottom.If you don't have any venting(cavitation) issues motor is probably too low.

 

[hwsiii]

Re: Correct prop for my Yam. 50 on a K.W.152CC

BK, like Spike says we need the GPS speed as well.

I need to know exactly which Yamaha SS prop you have on there right now, I have seen as much as 700 RPM difference between a 17" pitch Yamaha Painted SS that turns 5,750 RPM and the Yamaha Performance 16" pitch that could only turn 5,050, and the Painted Steel got 5.4 MPH higher speed as well.



H

 

[bklong]

Re: Correct prop for my Yam. 50 on a K.W.152CC

My GPS SOG is just under 30MPH. Motor is factory rigged by Key West, and the cavitation plate is at the correct height. Engine is running perfectly, with the throttle butterflies fully open at WOT. Only change I have made since it was new is to add a small...5"x10"... fixed trim tab to the port side of the transom. It is angled down at just a slight amount...maybe 5 degrees or so...to correct a chronic port side list while on plane, probably because the 13 gallon livewell is on that side at the stern.

 

[Lucraft]

Re: Correct prop for my Yam. 50 on a K.W.152CC

Just to add some info regarding bklong's situation.. (we're neighbors):

The motor is trimmed all the way down for the hole shot and is in excellent tune. We checked the throttle butterflies yesterday and all 3 are opening all the way at WOT. The hull is spotless and appears new. I drove the boat last week and it would only hit 4900-4950 RPM WOT after adjusting for the best trim on plane. There are no ventilation issues at 'normal' trim as best we can tell. GPS speed varied from 28.5 mph to 30 mph depending on wind direction and where we were in the boat. Nothing we could do would bring the RPM above 5000 at WOT.

I wondered if the trim tab would have that much affect on RPM. Bklong swears the motor used to turn at least 5300 with the same amount of weight in the boat, before the trim tab. For what it's worth, once on a plane the tab has very little effect on the flow of water when looking over the stern, and there is no excessive spray or anything from the tab to suggest that it's creating too much drag.

The only other issue this boat has ever had, (since new) is when cold cranking. The choke engages automatically as it should on the first start, but seems to disengage too soon and the motor dies after idling for about 10 seconds. Go to restart it and the choke engages again and this time it will idle all day without a problem. We're told the choke system on this motor uses some sort of linkage system that incorporates wax that melts once the motor reaches a certain temperature, disengaging the choke. This may be an unrelated tangent I'm going off on here, but is it possible the choke somehow isn't disengaging all the way, causing the low RPM at WOT? The motor runs absolutely perfectly otherwise and starts instantly once warm.

 

[steelespike]

Re: Correct prop for my Yam. 50 on a K.W.152CC

The early Matag front load washers had a "wax motor" that operated the door latch.
As it heated it would latch if you wanted to interupt the cycle to open the door it took a couple of minutes to cool enough to open.Yours must work saimilar but more related to the motor temp.Ours didn't fail much.
If your saying the this same rig used to do 5300 and doesn't now something is up with the motor or its the tab.Listing to port can also be caused by prop torque.
I think the tab has to cause some drag and also could cause you to have to steer
off center to combat the drag. Have you tried simply sitting slightly more to starboard or perhaps your allready there?I wonder how much weight was needed on the starboard side to combat the list it would seem that is what the tab is exerting on the water.

 

[bklong]

Re: Correct prop for my Yam. 50 on a K.W.152CC

I did try sitting all the way to starboard to correct the port list to very little avail. The trim tab has worked perfectly, tho. I agree on the possible torque steer issue on the listing prob. I do not now, however, have to steer the motor off-center to compensate for the trim tab. It's effect on steering is negligible.

 

[steelespike]

Re: Correct prop for my Yam. 50 on a K.W.152CC

As far as steering off center we're talking a few degrees not like that when fighting
a quartering wind.If you want to keep the tab as is I guess you need less pitch
Your looking for about 400 or 500 rpm We're talking a couple of inches.
11" sounds to me like that for a pontoon application.I don't know the gear ratio on your yammie but I know a typical 50 evinrude runs about 12 or 13" on a pontoon.